1. Field of the Invention
The present invention relates to a supersonic aircraft and method of operating the same, and more particularly to such an apparatus and method incorporating suction boundary layer control (and in some instances laminar flow control) in the wings of the aircraft.
2. Background Art
There are number of challenges in the design of supersonic aircraft that will meet performance requirements and yet be environmentally acceptable relative to community noise generation. For achievement of improved supersonic cruise efficiency, a preferred supersonic aircraft configuration employs highly swept (subsonic) leading edge wings. However, this design creates particular problems relative to high lift conditions typical of climb-out and approach where high angles of attack are required. More particularly, these highly swept wings develop two leading edge vortices which, while increasing lift, also result in an increase in drag, resulting in a poor lift to drag (L/D) ratio. Higher (L/D) ratio is obtained when there is fully attached flow over the wings.
In the Concorde supersonic transport, the wing has a highly swept leading edge, but no leading edge devices are used. During takeoff and climb, the configuration operates at a high angle of attack, and the two strong vortices that are generated off of the leading edges create sufficient lift for takeoff and climb. However, because of the high drag, the engines are operated at a relatively high power setting, thus creating noise well above the maximum level permitted in the vicinity of most all airports. Consequently, there are very few airports at which the Concorde can operate. Significant research effort has been directed at improving the L/D ratio of supersonic aircraft during takeoff and climb.
One prior art approach to obtain good high lift (L/D) performance is to employ leading edge devices such as flaps or slats, in order to maintain nearly attached flow. However, this approach is mechanically complex and may still lead to hinge line separation. Further, this requires additional hardware, and also the systems to operate the same, thus creating a penalty in both weight and cost. Additionally the space requirement for the accommodation of leading edge devices reduces the available fuel volume in the leading edge region of the wings.
A concept which has been proposed to generate increased left at liftoff and approach (desirably in combination with leading edge slats or other leading edge devices) is to utilize vortex generators in the form of apex fences located at the more forward inboard leading edge portions of the two wings. In this instance, during takeoff and initial climb, these fences are raised to generate two strong vortices that sweep over the inboard upper surfaces of the two wings to create increased lift during takeoff and initial climb. The drag created, however, requires a somewhat higher power setting for the engines (thus creating greater noise). As climb continues, these fences are moved to the stowed position to decrease drag so that the engine can operate at a lower power setting to decrease noise.
The subject of laminar flow control has been studied for a number of decades, and these studies have been reviewed in a recent publication, entitled "Fifty Years of Laminar Flow Flight Testing" authored by R. D. Wagner, D. V. Maddaion, and D. W. Bartlett, publication No. 881393 at the NASA Langley Research Center. There is discussed natural laminar flow (NLF) and also laminar flow control (LFC) which uses suction at the surface. Also discussed is HLFC (Hybrid Laminar Flow Control) which is said to be a ". . . flow control concept that integrates LFC and NLF and avoids the objectionable characteristics of each." Suction is applied at a forward location to obtain the LFC, and immediately aft of the LFC section natural laminar flow (NLF) exists. There have been proposals to incorporate a suction system for supersonic aircraft for cruise laminar flow control.
In a publication entitled "Application of Boundary Layer Control to HSCT Low Speed Configuration", AIAA/AHS/ASEE Aircraft Design, Systems and Operation Conference, Sep. 17-19, 1990/ Dayton, Ohio (one of the authors of this paper being P. H. Parikh, the inventor herein), there is discussed the feasibility of using boundary layer control (BLC) on a high speed civil transport (HSCT) high lift configuration for low speed performance improvement. This is shown as being incorporated in a supersonic aircraft having a double delta wing configuration where there is a highly swept (subsonic) forward inboard wing portion and a less swept (supersonic) outer rear leading edge wing portion. Leading edge flaps (specifically droop nose flaps) are provided on the outboard less swept leading edge wing portion. Laminar flow control suction areas are provided along the leading edge of the more swept inboard wing portions and laminar flow control suction areas are provided on the outboard wing portions in areas aft of the flap hinge lines. These LFC areas are provided to decrease drag during supersonic cruise.
As illustrated in FIG. 2 of this same article, boundary layer control suction is applied at location immediately aft of the hinge line of the droop nose flap, this being done to avoid separation of the air that flows upwardly and rearwardly over the upper surface of the flap and then travels in a curve to flow over the upper wing surface. Thus, this configuration employs a leading edge flap/BLC combination to avoid (or at least alleviate) the separated flow that would otherwise occur aft of the flap in certain situations.